Field of the Invention
The present invention mainly relates to an image forming apparatus such as a copying machine or printer of an electrophotography system or electrostatic storage system and, more particularly, to a control of a density and registration in an image forming apparatus.
Description of the Related Art
An image forming apparatus including a plurality of photosensitive members often causes relative misregistration between colors due to mechanical attachment errors of the photosensitive members, errors of optical path lengths of laser beams of respective colors, changes of optical path lengths, and the like. Also, image densities of respective colors vary depending on usage environments and various conditions such as the number of copies to be printed, thus causing a color balance variation.
For this reason, Japanese Patent Laid-Open Nos. 01-167769 and 11-143171 disclose an arrangement in which detection patterns as toner images used to detect misregistration amounts and densities are respectively formed on an intermediate transfer belt so as to correct the misregistration and densities. In these documents, misregistration and density detection patterns are detected by a single detection unit, thereby avoiding increases of a size and cost of the apparatus.
Japanese Patent Laid-Open No. 2001-166553 discloses an arrangement in which when misregistration and density corrections have to be successively executed, both misregistration and density detection patterns are formed on an intermediate transfer belt and are detected, thereby shortening a time required for correction control processing.
A sensor used to detect a density is controlled to be able to detect a density even when the intermediate transfer belt and light-emitting element deteriorate. By contrast, since misregistration detection uses a toner density in a detection pattern or a density difference between the detection pattern and the surface of the intermediate transfer belt, for example, when the density difference is small, misregistration often fails to be detected. When misregistration fails to be detected, process conditions (for example, a laser light amount, charging bias, developing bias, and the like) are changed based on the density detection result, and misregistration detection is restarted, resulting in a long correction control time.